Electrolytic processes



United States Patent 3,397,125 ELECTROLYTIC PROCESSES John Graham Tapley, West Kilbride, Scotland, assignor to Imperial Chemical Industries Limited, London, England, a corporation of Great Britain No Drawing. Filed Oct. 27, 1964, Ser. No. 406,872 (Ilaims priority, application Great Britain, Nov. 25, 1963, 46,441/ 63 5 Claims. (Cl. 204-3) ABSTRACT OF THE DISCLOSURE There is provided a process for refining metals selected from the group consisting of copper, silver, gold, zinc and cobalt, in which the improvement comprises electrodepositing the metals on an electrode having a surface of conducting silicone elastomer. The metal so deposited may be easily stripped from the electrode without contaminating the metal with parts of the electrode and without damage to the electrode. The electrode may, therefore, be used a plurality of times.

This invention relates to improvements in or relating to electrolytic processes.

In electrolytic deposition processes, for example, such as refining of metals or electroforming it is customary to use an electrically conducting substrate as an electrode. In these processes it is essential that the deposited material should be capable of subsequent easy removal from the substrate. This has not always been satisfactorily achieved hitherto. Various means have been adopted to overcome this difficulty including the use of various mould release agents but these means have not in all cases been entirely successful.

An object of the present invention is to provide a process for electrodeposition in which the deposited material can readily be removed from the substrate on which it has been deposited. Another object is to provide an electrically conducting substrate for use as an electrode in such a process. Other objects will appear hereinafter.

According to the present invention these objects are accomplished by a process of electrodeposition in which the electrically conducting substrate on which deposition takes place is formed of a synthetic elastomer.

Suitable electrically conducting synthetic elastomers of which the substrate for deposition may be formed include, for example, conducting butyl, chloroprene, butadienestyrene copolymer, butadiene-acrylonitrile copolymer and polysulphide rubbers and conducting silicone rubbers.

The hardness and flexibility of the substrate may be varied, for example, by alteration of the proportions of its ingredients, to give the desired final characteristics. Thus if a relatively friable granular material is being deposited, this may perhaps be best released from a soft very flexible substrate. On the other hand if a thick, coherent sheet of metal is being produced a tougher, almost rigid elastomer may give best release properties. The substrate may, if desired, be supported or reinforced, for example, by metal or fabric, for example, such as glasscloth, to give increased mechanical strength or electrical conductivity. In the case of materials of low mechanical strength such as silicone rubbers it is preferred to have a supported or reinforced electrode.

The elastomers used may be formed into the desired shape of substrate by the conventional methods of fabrication and vulcanisation or curing if necessary. The conductive properties may also be obtained in known manner, for example, by the use of conducting fillers such as carbon blacks or metal powders. The conductivity of the material of the electrode will normally be adjusted to that most suitable for the particular electrolysis conditions under 3,397,125 Patented Aug. 13, 1968 which it is to be used. It will, however, normally have a volume resistivity not greater than ohm-cm.

While a wide variety of electrically conducting synthetic elastomers can be used it is in many cases preferred to use a conducting silicone rubber. This rubber may be formed from a heat curable composition or from a room temperature curable composition and may be shaped by, for example, moulding, extruding, spraying on to a textile or metal fabric or other known means. Such conducting elastomer-forming compositions are well known and are readily available.

While the substrates used in the process of my invention have adequate release properties these may be further enhanced, if desired, by the application of a release agent such as a silicone fluid prior to electrodeposition.

Materials which may be electrodeposited using the process of my invention include metals such as copper, silver, gold, zinc and cobalt, and other similar materials.

My invention is further illustrated by the following examples in which all parts and percentages are by weight except where otherwise stated.

Example 1 The following ingredients were compounded together: Parts Methylvinylpolysiloxane (0.1 mol. percent vinyl groups, average M.W. 750,000) 100 Acetylene black (average particle size 42 m surface area 65 m. /g.) 65 Fume silica (coated with 20 percent octamethylcyclotetrasiloxane) (average particle size 1040 m surface area 175 m. /g.) 10 Methylhydrogenpolysiloxane (average M.W. 2000,

H/Si ratio 0.96/1) 5 Platinum in form of cyclohexene/platinous chloride complex 0.004

The composition so obtained was formed into a sheet and subjected to a pressure of 500 lb./sq. in. at 170 C. for 15 minutes to give a cured sheet 0.25 thick. This sheet was then heated at 250 C. for 24 hours in an air circulating oven.

The physical properties of the cured sheet were measured using the methods described in British standard specification 903 of 1956 and were found to be:

Elongation at break percent. Tear strength 6 lb. Tensile strength 525 lb./sq. in. Hardness 84 13.8.

The electrical resistivity when measured by the method of British standard specification 2,044 of 1953 was found to be 4 ohm-cm.

A portion of the sheet so prepared was then used as the cathode in a bath for the electrodeposition of copper. The bath contained 40 g./l. copper as copper sulphate and 200 g./l. sulphuric acid. A sheet of copper in the bath and parallel to the cathode formed the anode. A direct current at a current density of 20 amps/sq. ft. at the cathode was passed through the bath for 5 hours, the bath being stirred continuously to prevent polarisation. The deposited sheath of copper resulting from the electrolysis was easily peeled from the silicone rubber cathode leaving the rubber with no visual damage and with only a trace of black on the copper sheath.

The process of electrodeposition and release was repeated five times after which the rubber was examined for damage. No damage was visible and no decrease in ease of release was apparent.

The thickness of the copper sheath tended to taper otf as the distance from the electrical connection to the silicone rubber sheet increased due to the appreciable resistance of the rubber.

Example 2 The formation of a sheet of conducting silicone rubber as described in Example 1 was repeated except that the composition was pressed into a sheet in intimate contact with a clean, degreased zinc plate. There was thus obtained a cured silicone rubber sheet firmly bonded to a zinc plate over its whole surface. The electrical efiiciency of the bond was checked by measuring the resistance between the zinc plate and spots on the exposed surface of the conducting rubber using a probe of diameter 1 mm. Resistance variations of only a few percent were found.

The zinc supported rubber was used as the cathode in a copper deposition bath in the manner described in Example 1, the zinc plate being protected from the action of the bath by a heavy layer of insulating silicone grease. The deposited copper was easily stripped from the silicone rubber sheet. Five successive deposition and release cycles were then carried out after which it was found that there was no visible surface damage to the rubber or breakdown of the bond between the rubber and the zinc plate. There was also no decrease in the ease of release.

In this case deposition was much more even than in Example 1. The metal backed cathode was also much stronger mechanically and could be bent into a variety of stable configurations.

After compounding the mixture was dispersed in toluene to give a dispersion containing 30 percent solids.

1.5 percent of dibutyl tin dilaurate was incorporated in the dispersion which was then painted on a thin aluminium sheet. The painted film was allowed to dry and cure at C. for 24 hours.

The aluminium sheet coated with conducting silicone rubber was then electroplated with copper in the manner described in Example 1. After three days the copper, which parted very readily, was peeled off the rubber coated sheet. The copper showed no visible signs of contamination by silicone or carbon.

Example 4 A dispersion containing 20 percent of the composition as described in Example 1 dispersed in toluene was prepared.

A portion of woven glasscloth (desized) approximately 5 thousandths of an inch thick was dipped in the dispersion, removed therefrom and allowed to drain and dry in warm air for 1 minute at C. The dried but uncured rubber film so produced was then cured by heating to C. for 15 minutes. The resulting coating had a thickness of approximately 2 thousandths of an inch and exhibited a surface resistivity of 260 ohms/sq. cm.

The glasscloth-supported rubber was used as the cathode in a copper deposition bath in the manner described in Example 1. As the cathode had a relatively high resistance a current density of only 0.36 amp/sq. ft. was used. After 24 hours when an initial covering of copper had been deposited the current density was increased to 1.4 amps/sq. ft. and the process continued for a further 72 hours. The deposited sheath of copper resulting from the electrolysis was very easily peeled from the glasscloth/ silicone rubber cathode, leaving the rubber with no visible damage and only a trace of black on the copper sheath. The force required to peel the flexible glasscloth/silicone rubber cathode from the copper sheath was measured at a parting rate of 20 inches per minute and was found to be approximately 300 grams per inch width.

What I claim is:

1. In a process of refining a metal selected from the group consisting of copper, silver, gold, zinc and cobalt, the improvement comprising electrodepositing the said metal on an electrode having a surface consisting essentially of an electrically conducting silicone elastomer, whereby the so deposited metal may be easily stripped from the said surface without contamination of the deposited metal or damage to the said surface, and the said electrode may be reused a plurality of times.

2. A process according to claim 1 wherein the substrate is reinforced or supported by metal or fabric.

3. A process according to claim 2 wherein the substrate is supported by a metal plate.

4. A process according to claim 2 wherein the substrate is reinforced by a glasscloth fabric.

5. The process of claim 4 wherein substrate support by a metal plate is bent into any desired configuration.

References Cited UNITED STATES PATENTS 859,813 7/ 1907 Hartmann 204-20 1,577,981 3/1926 Otto 204--20 X 2,732,020 1/1956 Scholl -159 2,768,133 10/1956 Lundbye 204--20 FOREIGN PATENTS 147,077 9/ 1962 USSR. 147,078 9/1962 USSR.

OTHER REFERENCES Condensed Chemical Dictionary, sixth edition, Reinhold Co., p. 1019 relied on.

JOHN H. MACK, Primary Examiner.

W. VAN SISE, Assistant Examiner. 

